Passive InterModulation (PIM) distortion is a form of electrical interference that may occur when two or more radio frequency (RF) signals encounter non-linear electrical junctions or materials along an RF transmission path. Such non-linearities may act like a mixer causing the RF signals to generate new RF signals at mathematical combinations of the original RF signals. These newly generated RF signals are referred to as “intermodulation products.” The newly generated RF signals may fall within the bandwidth of existing RF signals. This may occur, for example, when signals transmitted through a device generate intermodulation products that fall in the same bandwidth of signals that are received through the same device. If this occurs, the noise level experienced by the existing RF signals in the receiver bandwidth is increased. When the noise level is increased, it may be necessary to reduce the data rate and/or the quality of service. PIM distortion can be an important interconnection quality characteristic, as PIM distortion generated by a single low quality interconnection may degrade the electrical performance of the entire RF communications system. An unexpected current from an outer conductor of a cable in the antenna may increase PIM distortion levels and/or influence the isolation stability of the antenna.
The unexpected current may occur in an unshielded circuit that is included in a feed network of the antenna. The unshielded circuit may be, for example, any element made of microstrip or printed circuit board materials that is capable of radiating outwards.
For example, the unshielded circuit may be a power divider or a phase shifter. A plurality of cables may be attached to the unshielded circuit. For example, if the unshielded circuit is a phase shifter, an input cable and a plurality of several phase cables may be connected to the unshielded circuit. An unexpected current may appear on an outer conductor of one of these cables.
An unexpected current may also or additionally occur around a radiating element of the antenna. Usually, each radiating element is connected to a reflector of the antenna, which serves as a ground plane, and is also connected to an unshielded circuit via a cable. When performing service and maintenance work, technical personnel may separate the radiating element from the reflector, and thus the radiating element may no longer be connected to ground. In this situation, for example, the unexpected current may leak through the outer conductor of the connecting cable.